When the airline sends outbound international flight passenger manifests to Homeland Security, a form of exit clearance happens behind the scenes in the sense that persons traveling on international passports are 'logged out' of the country electronically upon the transmission of that document rather than DHS having to pay for people to physically do so at the airport.

There's actually a different form that non-Canadians and non-Americans supposed to use if exiting the USA at a land border where there isn't that kind of passenger manifest to note your departure.